A rail-rail comparator input stage has two types of differential-pairs that have to be offset calibrated separately. This requirement complicates the addition of precision hysteresis to the rail-rail comparator because the Gm (transconductance) of the rail-rail comparator input stage varies with input common mode voltages. As a result the hysteresis voltage becomes a variable, especially when the input common mode voltage is near the middle of the voltage rails the hysteresis becomes half the value near either rail, e.g., Vdd or common (ground).